Grounding of Generators

In rotating electrical machines like motors and generators, if the earth fault current is high, as in the case of solid earthing, the core damage would be high. To limit the damage to the core, machine manufacturers allow only a limited ground fault current. This is given in the form of a core damage curve. A typical value would be 25A for 1 second. This value is used as a guide in selecting NGR and setting stator earth fault relays in generator protection.

Winding damage in rotating electrical machines is not of serious concern. The repairs to winding damages can be done by the local re-winder. But, in case of core damage, repairs cannot be carried out at site. The machine has to be sent back to the manufacturer's works for repairs thus resulting in prolonged periods of loss of production.

Since rotating electrical machines are not present in voltage levels from 22kV onwards, these systems are usually solidly grounded. At EHV level too, solid grounding is universally adopted for two reasons:

- Cost of insulation at EHV level is significant.

- Primary protection in such systems would clear the fault within 5 cycles.

If rotating machines are present in 3.3kV, 6.6kV & 11kV levels, the systems are grounded through resistor or reactor to limit the earth fault current. If rotating machines are not present at these voltage levels, then these systems can be solidly grounded.

In case of LV Systems, though rotating electrical machines are present, the system is solidly grounded to confirm to IE Rules. [Rule 61 (1) e]. Since LV System is also handled by general public, for safety reasons, solid grounding is mandated. Sufficient ground fault current is allowed to flow so that protective devices can operate and clear the faults at the earliest. Of course, the core damage at the point of fault in rotating machines will be high.